This invention is related to an initial core of a Boiling Water Reactor (BWR) disposed in a nuclear power plant.
Generally speaking, in a conventional BWR, the average enrichment of fuel assemblies in an initial core is the same for all fuel assemblies. In a nuclear reactor, 1/3-1/4 of all fuel assemblies of are taken out at every operation cycle and they are replaced by new fuel assemblies. But, the average enrichment of fuel assemblies for the initial core is set to such a value that the burning of the fuel assemblies is possible for a period of 2-3 cycles (in the following, this is referred to as "the first cycle"). At the end of the first cycle, it is uneconomical, in the exchange of the fuel assemblies, for the fuel assemblies which are not burned enough and which contain a large amount of uranium 235 to be taken out of the core.
Thus, after the first cycle, fuel assemblies are changed partially and the operations are continued. These operation cycles are referred to as "the second cycle", "the third cycle", etc. New fuel assemblies which are loaded at the beginning at second cycle or of the later cycles are called reload fuel assemblies.
The core in which reload fuel assemblies are loaded consecutively over several operation cycles after the first cycle eventually reaches a stable core state. That is to say, the thermal characteristics of both the previous cycle and the next cycle are almost the same and the operation cycle becomes stable with respect to the thermal characteristics. This operation cycle is called an equilibrium cycle. The core which has reached an equilibrium cycle is called an equilibrium core.
In such a nuclear reactor, it is preferable for the thermal characteristics and cycle exposure in intermediate cycles (in the following, referred to as a "transition cycle") between the first cycle and the equilibrium cycle to be substantially the same as those of the equilibrium cycle, or for the transition cycle to reach the characteristics and exposure of the equilibrium cycle as soon as possible. However, like the conventional initial core, in the case where the average enrichment of fuel assemblies is one and the same level, a long time is necessary for changing from the transition cycle to the equilibrium cycle and a change of all the number of reload fuel assemblies in the transition cycle is large.
For this reason, in a BWR, it has been suggested to combine various kinds of fuel assemblies having a different average enrichment to constitute an initial core. Fuel assemblies of lower average enrichment are taken out in such an arrangement after one operation cycle and are replaced with new fuel assemblies so that the average discharge exposure of the fuel assemblies loaded in the initial core is increased and a quick transition to the next cycle is carried out. This technology is described in Japanese patent laid-open No. 57-8486, for example.
On the other hand, for extension of an operation cycle and for high exposure, it is necessary to increase the average enrichment in the initial core. As mentioned above, when various kinds of fuel assemblies having a different average enrichment are combined into the initial core, the difference of enrichment among the fuel assemblies becomes large, and the difference in nuclear properties between high enrichment assemblies and low enrichment assemblies becomes large.
When the fuel assemblies having largely different nuclear properties adjoin each other, each fuel assembly radiates neutrons to adjacent fuel assemblies because of the different neutron spectrum of the respective fuel assemblies. As a result, in the early stage of burning, the Maximum Linear Power Heart Generation Ratio (MLHGR) becomes large and the thermal margin of the nuclear reactor core becomes small. Thus, a technical problem which arises with this core is to improve the thermal margin.
When the average enrichment of the core is increased, there is a tendency for excess reactivity in the core to become larger. Therefore, it is necessary to increase the number of control rods in the core during operation. In this case, when fuel assemblies of control cells, into which control rods are to be inserted, are burnt as a result of control rods being inserted into the control rods for a long period of time, the fuel assemblies have a smaller power at the inserting side of the control rods, but a larger power at the opposite side. Because burning of fuel rods at the inserting side is slow, there is a phenomenon that a power of the fuel rods at the inserting side becomes large when the control rods are pulled out. Therefore, fuel assemblies surrounding the control rods inserted during the operation for a long period of time are fuel assemblies having the lowest enrichment (low enrichment assemblies). In this way, in the initial core which has the objective of high exposure, it is necessary to arrange a lot of control cells, each having a control rod surrounded by four low enrichment assemblies.
Therefore, there is a tendency for the number of high enrichment assemblies to become high in fuel assemblies loaded in cells other than the control cells, and hence it is necessary to provide as many high enrichment assemblies as possible. But, since the enrichment of the high enrichment assemblies is high, when high enrichment assemblies are arranged adjacent to one another, the power of these fuel assemblies becomes higher and there is a tendency for the thermal margin of the nuclear reactor core to become severe.